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16 tooth speedo gear tire size accuracy; speedometer accuracy
Topic Started: Jan 23 2014, 03:02 PM (1,262 Views)
doihaveto
New Member
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Here is a list of 14 inch sizes with the 13 stock equivalent with stock speedo gear and the accuacy percent etc...

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
155/80-13 4.9in 11.4in 22.8in 71.5in 886 0.0%
185/55-14 4.0in 11.0in 22.0in 69.2in 916 -3.3%
185/60-14 4.4in 11.4in 22.7in 71.4in 887 -0.1%
175/65-14 4.5in 11.5in 23.0in 72.1in 879 0.8%

I straightened the first line to make the categories line up should figure out the rest from there cause I had just cut and pasted it all

So I went from 13stock 155/80-13 to in my case 175/65-14 because it is what they had at the time used

As you can see it should make stock speedo off by +0.8% or 0.08 but because of already being partially worn it is actually off right now by +0.5% or 0.05 and will get closer and closer to accurate as they wear down

If I was to keep stock speedo gear and get 185/60-14s my accuracy would be -0.1 or 0.01 but get farther from accurate as they wear not closer.

My question is...does anyone have the 16 tooth gear in the tranny(manual) while running 14" tires and what size tires are you running with the 16 tooth gear to get as close to +-0.0% accuracy

I already own the 16 tooth gear so I do not have to buy it no need to say just run the 180/60-14s and forget the gear swap

Since the calculator used above works off of full tire tread I would rather have +0.4% to -0.4% accuracy because it would get closet towards accurate as it wears rather than the other way since obviously the stock tires get farther from accurate as they wear.

I will take anyone weighing in on this but really would like to hear from anyone who has he 16 tooth gear running 14 inch wheels already...

Thank you all.
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doihaveto
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Sorry when I actually posted the chart was no longer lined up
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Old Man


gear tooth wear has nothing to do with accuracy because the wear does not change the RPM of the gear. It stays the same no matter how much wear occurs---at least til the gear teeth are worn away, then you have no accuracy at all.
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Coyote X


On my convertible car I had 175-70/14 tires with the 16 tooth and the speedo was about as accurate as any Metro. I am not sure how well they fit into the stock wheel wells though mine was far from stock so I don't know where they might rub.
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Deleted User
Deleted User

Just about the time you think you have it figured out to the 1/10th of a percentage, Geo Glenn comes along and points out that the diameter of your tire will decrease 1/2" from the time it's new off the rack, until the time it hits the 2/32" warning band. :(

Pick a set of tires that you like for reasons that make sense to you. :coffee

For instance, there's a nice pair of used 175/70R13's that I'm eyeing at the local used tire place. 7/32" tread remaining. NHTSA UTQG = 420AA. Tire guy says $60 for the pair, mounted and balanced. For that price, I really don't care to split hairs on the percentage over or under theoretical standard. :news

If you're really worried about how fast you're going, or how far you've been, get a GPS. ^o)

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Old Man


my speedo goes up and down so fast and so much I don't pay a lot of attention to it :lol
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Woodie
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You're looking for far more accuracy than is available. Buy the same size tires from five different manufacturers and you'll get five different results. Then, they'll change drastically from new to worn.


Edited by Woodie, Jan 24 2014, 06:00 AM.
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sphenicie


Geo Glenn
Jan 23 2014, 08:54 PM
Just about the time you think you have it figured out to the 1/10th of a percentage, Geo Glenn comes along and points out that the diameter of your tire will decrease 1/2" from the time it's new off the rack, until the time it hits the 2/32" warning band. :(



hey Glenn, I am not meaning to be picky to "1/10th of a %", but, I do wonder about the ' 2/32" '. :evillol is there a particular reason for stating it in 32nd's?

I am kinda partial to 0.0625" myself, +/- 6.25 x(10)-5, of course! :evillol :oshit :evillol
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doihaveto
New Member
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I guess I should have specified that it is not the speed accuracy but the odometer miles is my reason I need that accuracy for my work...even 0.01 becomes 100 over 100000 or 0.08 800 over 10000...and i know most tires are only 45 to 60 thousand mile tires

But I will take 0.01 or even 0.08 but was just wondering what others may have or been using

Ideal would theoretically be to start at about + 0.0_ and the _ being 3 or 4 or 5 and by the time you end up at the 2/32... (1/16) lol for the fraction police...you are at - 0.0_ again being 3 4 or 5 so the plus and the - then offset.

I know you are giving geo glenn a hard time but i hope we all know why he used 2/32 but that post was amusing when I saw it and made me think what if we did use proper fraction on tire wear...wouldnt that get frustrating

But thank you geo Glenn...I brought up the topic here because the tire size you gave me a few weeks ago I did not find in my area readily available...so I figured I would see who might have any ideas or experience around the forum

But thanks a also to the rest of you for your 2 ¢ worth

I will take any more suggestions any might want to pop in with and will at least at some point make sure it finish on this topic with what I end up running in the end and stick with

Thanks again
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Metromightymouse
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Powdercoat Wizard

If the accuracy is that important for your job, or whatnot, it would seem that a GPS would be the most consistent solution.
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davegran
Old Fart w/Wrenches

Metromightymouse
Jan 24 2014, 09:49 AM
If the accuracy is that important for your job, or whatnot, it would seem that a GPS would be the most consistent solution.
Doesn't have to be a fancy one, either.... B-)

Posted Image
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myredvert
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myredvert

Quote:
 
hey Glenn, I am not meaning to be picky to "1/10th of a %", but, I do wonder about the ' 2/32" '. :evillol is there a particular reason for stating it in 32nd's?
US tire tread wear has historically (at least since the 60's) been measured in x/32nds. Tire wear gauges are marked this way. Average new passenger car tires usually have between 10/32 and 12/32 of tread.

Standard wear bars (and the top of Lincoln's head on a penny) are 2/32"
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Deleted User
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I remember we were talking about tire sizes a while back, and availability of the 165/65R13 seems to be a deal killer for this old time favorite.

The Kumho Tires Solus KR-21 in size 165/70R13 78S BW A/S, are around 22" in diameter to start, and will, of course, get smaller over time.

I'm most certainly considering them for my 1990 LSI with the XFI engine resurrection that will soon be installed.

http://www.discountedwheelwarehouse.com/Discount_Tires.cfm?pn=KT-2113073&pID=45504

UTQG: 640 A B
Warranty: 85,000 Mile

This is an outstanding tire according to the ratings and reviews. It's the next one in line after the Michelin Defender, except the Defender's 175/70R13 size is a bit of a stretch to use on a true XFI, perhaps.

I'll try a brown speedometer gear, and a while speedometer gear. Whichever is the closest without going over wins. (The Price is Right is on the TV in the other room. :hmm )

I've been doing GPS miles per gallon for a very long time. You'll get a little variation going up and down hills, perhaps. I don't worry about it too much. It's a system, and it's very repeatable.

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doihaveto
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Yes the GPS is a great idea and I have used it before back when I had an auto trans with bad speedo gear inside the transmission the gear inside not the gear on the end of the speedo gear we can take off and swap was in pieces in the bottom of the transmission fluid pan.

Anyways my issue there was that I forgot to start the GPS up to turn it on of to record the miles etc

Considering the options with tire size, speedo gear, etc before I go back to the gps option

If I can be inconvenienced for a little while now to get it right then it will be right so long as i buy the same tires

The GPS option will work but got to remember it all the time it becomes a constant inconvenience.

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Metromightymouse
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Powdercoat Wizard

Ought to be one or more out there with an auto on feature.
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